Sensor for helium or hydrogen

ABSTRACT

The invention relates to a sensor ( 1 ) for helium or hydrogen with a vacuum-tight housing ( 2 ), wherein a gas consuming cold cathode array ( 3, 4 ) is arranged, in addition to a selectively active passage ( 9 ) for the gas to be detected. According to the invention, in order to improve the properties of said sensor, the housing ( 2 ) is made of glass, components of the gas passage ( 9 ) include a membrane ( 18 ) made of a silicon material having the desired selective qualities, in addition to a silicon plate ( 19 ) which supports the membrane ( 18 ) and is provided with a plurality of openings ( 21 ) and a heating element ( 24 ). The housing ( 2 ) and the selectively active gas passage ( 9 ) are joined together without polymer and elastomer.

[0001] The present invention relates to a sensor for helium or hydrogenwith a vacuum-tight housing, wherein a gas consuming cold cathode arrayis arranged, in addition to a selectively active passage for the gas tobe detected.

[0002] From DE-A 43 26 265 a sensor having these characteristics isknown. Sensors of this kind are of a relatively simple design and may beoperated approximately up to atmospheric pressure (outside theirhousing). Still, sensors of this kind have not been able to penetratethe market in particular because they are, compared to the commonlyemployed but more costly mass spectrometers, too insensitive and can forthis reason not be employed under vacuum conditions at low partialpressures of the test gas.

[0003] It is the task of the present invention to create a sensor of thekind mentioned above having improved properties. This task is solvedthrough the present invention by the characterising features of thepatent claims.

[0004] In that the housing is made of glass, H₂ gas is not outgassedwithin the housing. Preferably borosilicate glass is employed since itis suited for both fusing with correspondingly thermally adapted metalsas well as anodic bonding or diffusion welding with silicon. The passage(as is basically known from DE-A-195 21 275) employed in the sensoraccording to the present invention permits the arrangement of a gaspassage having a relatively large surface area of sufficient stability.Finally the joint of the gas passage with the housing by means of anodicbonding which is free of polymer and elastomer offers the advantage thatno outgassing into the inside of the housing is effective originatingfrom seals or adhesives. All these measures are beneficial with respectto the sensor's sensitivity resp. result in an improved detection limit.

[0005] Further advantages and details of the present invention shall beexplained with reference to the examples of embodiments depictedschematically in drawing FIGS. 1 to 4. Depicted are/is in

[0006] drawing FIGS. 1 to 3 an embodiment with a substantially cuboidhousing and

[0007] drawing FIG. 4 an embodiment having the geometrical arrangementof a magnetron.

[0008] In all drawing figures the housing of the sensor 1 is designatedas 2, the cathode components accommodated within the housing 2 as 3, andthe anode components as 4, and the magnets located outside of thehousing 2 are designated as 5. The otherwise closed housing 2 is open onits respective face sides 6. In this area it is equipped with aflange-like rim 7 the annular surface 8 on the face side serving thepurpose of providing the joint between the gas passage 9 (only depictedin drawing FIGS. 1 to 3) with the housing 2 by means of anodic bondingor diffusion welding. The face side 11 of the housing 2 opposite theopen face side 6 resp. the gas passage 9 is equipped with electricfeedthroughs 12 and 13 which are linked via lines to a control,measuring, recording, display and/or power supply unit depicted only asblock 14. Feedthrough 12 serves the purpose of supplying the cathodecomponents 3 with high tension. Feedthrough 13 links the anode 4 to theunit 14. Said feedthrough 13 is equipped with a shield 15, since thefed-through line serves the purpose of supplying extremely low ioncurrents (down in to the fA range) to unit 14. As the materials for thefeedthroughs 12, 13 and for shield 15, iron (cobalt)/nickel alloys(trade names VACON, KOVAR, . . . ) have been found to be the mostexpedient. Tungsten and platinum may under special circumstances also beemployed together with special types of glass.

[0009] In the embodiments depicted in drawing FIGS. 1 to 3 the housing 2has substantially the shape of a cuboid. Located within the housing 2are an annular anode 4 made of stainless steel and two approximatelysquare cathodes 3 preferably made of titanium sheet sections which arefitted in front of the face sides of the annular cathode 3 in the usualmanner. For the purpose of minimising degassing of hydrogen, theelectrodes have been annealed at approximately 600° C. to 900° C. in ahigh-vacuum degassing process.

[0010] Drawing FIG. 3 is a perspective view. The housing wall facing theviewer is missing. The magnetic field produced by the magnets 5,preferably permanent magnets, is indicated by arrows 17.

[0011] Drawing FIG. 3. comprises two enlarged views 3 a and 3 b for thegas passage 9. Drawing FIG. 3a depicts a sectional view through the gaspassage 9. It is composed substantially of a membrane 18 with thedesired selective properties and a carrier 19. The carrier 19 is asilicon plate, which is provided with a plurality of openings 21produced with the aid of known etching methods. The openings 21 formwindows closed by the membrane 18 said windows having a surface area of0.5 mm², for example. The sum of the window surfaces forms the actualgas passage (total penetration area).

[0012] The gas passage 9 may, for example, be manufactured based on aprocess as is known from DE-195 21 257 A1. The same also applies to theheater 23 with which the gas passage 9 is equipped for the purpose ofincreasing the response time. Drawing FIG. 3b depicts an embodiment inwhich each of the window surfaces is equipped with a heating filament onthe outside, said filament made of nickel/chromium or preferablyplatinum 24. The heating filaments are supplied with electric power fromthe outer rim of the structure where on each narrow side one pole ofeach of the heating filaments is joined to the others.

[0013] In the embodiment in accordance with drawing FIG. 4¹⁾ the housing2 and the cathode 3 are designed to be cylindrical in shape.

[0014] The anode 4 has the shape of a rod which is arranged along thejoint axis 25 of housing 2 and cathode 3. The lines of magnetic fluxproduced by the magnet 5 are marked by arrows 26.

[0015] Through the measures in accordance with the present invention itshall be achieved that no degassing is effected into the housing 2, soas to attain a level of partial pressure sensitivity comparable to massspectrometers. Since it is required to match resp. provide a tight sealbetween the housing 2 made of, for example, borosilicate glass and othermaterials, it is expedient that different areas of the housing 2 be madeof different types of glass having different coefficients of expansion.Thus there exists the possibility of adapting the coefficient ofexpansion of the housing at the point where it needs to be joined to another material to the coefficient of expansion of the other material.The sealing properties of the housing 2 are thus improved, materialtensions are reduced.

[0016] For the purpose of adaptation to silicon having a thermalcoefficient of expansion of 3.0 ppm/K, DURAN glass at 3.3 ppm/K isespecially well suited. For matching to KOVAR or VACON, SCHOTT fusingglass 8250 at 5.0 ppm/K will provide an optimum solution. Besides this,matching platinum with thermometer glass is possible and in principletungsten can be matched to DURAN. For the purpose of matching thedifferent coefficients of expansion, a range of so-called transitionalglass types is being offered which need to be employed in propersequence when aiming at avoiding temperature induced strains and thusavoiding leaks or even damage.

[0017] In the embodiments depicted, for example the type of borosilicateglass employed in the area of the face side 6 resp. the anodic bondingsurface 8 for the housing 2 is matched to the coefficient of expansionof the silicon plate 19 carrying the selective membrane 18. In the areaof the opposing face side 11, the type of borosilicate glass employedthere for the housing 2 has expediently a coefficient of expansion whichcorresponds to that of the metals (KOVAR or VACON, for example) fedthrough at that point. In the transitional area between the face sides 6and 11, a transitional type of glass is expediently employed having acoefficient of expansion between the two coefficients of expansion ofthe two types of glass which are employed at the respective face sides.If necessary also several transitional types of glass, the coefficientsof expansion being graded in the same direction, may be employed for thepurpose of avoiding strain within the materials. There thus result thefollowing alternatives for production:

[0018] Housing made of DURAN glass (SCHOTT), current feedthroughs madeof KOVAR or VACON fused in SCHOTT 8250 glass and matched with domes oftransitional glass to DURAN, silicon disk with direct anodic bonding;

[0019] Housing of fused glass (SCHOTT 8250), current feedthroughs madeof KOVAR or VACON, silicon disk with intermediate aluminium layer,diffusion welded;

[0020] Housing made of DURAN glass (SCHOTT), current feedthroughs madeof tungsten, silicon disk with direct anodic bonding;

[0021] Housing made of DURAN glass (SCHOTT), current feedthroughs madeof platinum fused in thermometer glass and matched with domes oftransitional glass to DURAN, silicon disk with direct anodic bonding.

[0022] The types of glass stated are only to be taken as examples,decisive is in each instance matching of the coefficients of expansionbetween metal and glass.

[0023] The benefits of the sensor in accordance with the presentinvention are in particular that these may be operated approximately upto atmospheric pressure are that they are sufficiently sensitive. Forthis reason they may be employed instead of mass spectrometers for thepurpose of detecting leaks, i.e. they may be a component of a test gasdetector of leakage test instruments, independently of whether a testsample is connected to the leakage detector or the provision of asniffer employed to scan the test object.

[0024] Also the employment in vacuum systems, for example involvingcontinuously changing pressures is beneficial both as being a componentof a test gas detector of the leakage detection facility or as a gassensor in general. When searching for leaks on a vacuum chamber it issprayed from the outside with the test gas. In the instance of aleakage, the test gas enters and is recorded by the sensor.

1. Sensor (1) for helium or hydrogen with a vacuum-tight housing (2),wherein a gas consuming cold cathode array (3, 4) is arranged, inaddition to a selectively active passage (9) for the gas to be detectedwherein the housing (2) is made of glass, components of the gas passage(9) include a membrane (18) made of a silicon material having thedesired selective qualities, in addition to a silicon plate (19) whichsupports the membrane (18) and is provided with a plurality of openings(21) and a heating element (24) and where the housing (2) and theselectively active gas passage (9) are joined together without polymerand elastomer.
 2. Sensor according to claim 1, wherein the housing (2)and the selectively active gas passage (19) are joined together bybonding or diffusion welding in a high-vacuum tight manner.
 3. Sensoraccording to claim 1 or 2, wherein the cathode of the cathode arrayconsists of titanium plate.
 4. Sensor according to claim 1, 2 or 3,wherein the housing (2) consists of borosilicate glass.
 5. Sensoraccording to one of the claims 1 to 4, wherein the side facing away fromthe gas passage is equipped with voltage or current feedthroughs. 6.Employment of a sensor having the characteristics of one of the aboveclaims for the purpose of leakage detection.
 7. Kind of employmentaccording to claim 6, wherein the sensor is a component of the test gasdetector of a leakage detection instrument.
 8. Employment of a sensorhaving the characteristics of one of the claims 1 to 5 in a vacuumsystem.